Friday, August 12, 2011

Death of the World's Fastest Airplane*

The Defense Advanced Projects Research Agency (DARPA) launched the second and last of its HTV-2 hypersonic test vehicles yesterday, in what some media outlets have labeled "the world's fastest airplane." It's true that the HTV-2 skirted the atmosphere at Mach 20 once separated from its Minotaur IV booster rocket, so it might be the world's fastest unpowered glider. But the same could be said about the space shuttle. Like the shuttle, HTV-2 maneuvered while in a hypersonic glide, partly to bleed off excess airspeed.

Disappointingly, HTV-2's flight ended about 10 minutes short of what was expected. After 20 minutes, DARPA reported that they had lost telemetry. The most likely scenario? Somewhere during gliding flight, the vehicle disintegrated. Even after 30 years of shuttle flights and countless other rocket-plane programs before that, controlled hypersonic flight is still difficult. Unlike a ballistic vehicle like a space capsule, a winged vehicle has to maintain active control during flight. The aerodynamic stresses at such extreme speeds are high, and the unbelievable heating of the aircraft's structure causes it to lose much of its inherent strength. Even a small deviation from a safe flight attitude can destroy the aircraft. This is what happened as space shuttle Columbia began to encounter increased drag on its damaged left wing, with the orbiter's thrusters trying in vain to correct the unexpected yaw until they could no longer prevent the shuttle from breaking apart. DARPA will probably reach a similar (albeit more detailed) conclusion after analyzing all of their telemetry from the mission.

Several years ago, HTV-2 was supposed to be a stepping stone to what the defense department called "FALCON," or Force Application & Launch from Continental US. The idea was to strike a time-sensitive target so rapidly that there would be no way to avoid the attack. But existing ballistic missiles weren't acceptable because they would startle Russia or China into thinking we were launching the nuclear ICBM's. FALCON aimed to create hypersonic gliders and eventually a hypersonic bomber to meet the goals of rapid global strike. Apparently a straight-up launch of the glider on a converted ICBM (like Minotaur IV) would not give the other nuclear powers a false alarm. But now the idea appears to be dead, for lack of results and funding.

The basic idea of a hypersonic glider has been studied exhaustively since the 1950's. As the whims of the military and political leadership change, the FALCON concept may get a fresh look.